Impact processes are encountered when bodies are subjected to rapid impulsive loading, where the duration of application is short compared to the time for the body to respond inertially. The inertial responses are stress pulses propagating through the body to communicate the presence of loads to interior points. Commonly, such loadings are the result of ballistic impact or explosion.
Armors for the protection of personnel and equipment against impact processes are an area of significant effort. Armors are often multi-layer protective systems, with distinct protective characteristics arising as a result of individual material characteristics and resulting interfaces. In a typical armor system, the kinetic energy of an incoming projectile or blast wave is dissipated through deformation or destruction of a front plate with backing plates providing for subsequent dissipation of kinetic energy that may transfer from or pass through the front plate without absorption. Typically, if an impact drives a material beyond its elastic strength, then an elastic wave behaving as a shock wave propagates away from the impact zone with an amplitude determined by the largest elastic stress that can be supported by the medium. Behind this wave there propagates a generally irreversible deformation wave that carries the material to the ultimate stress state that exists on the impact plane. The energy of the elastic precursor wave is generally much less than the subsequent deformation wave, however when transmitted and coupled to a human body, the elastic precursor wave can result in significant trauma.
As an armor component, hydrogels have been investigated as energy absorbing components. Hydrogels have been utilized in various armors and blast protections both as primary absorption mechanisms and as backing layers. Hydrogels generally are cross-linked polymer networks having hydrophilic properties. When immersed in water, water diffuses into the hydrogel network due to osmotic pressure differences. The extent of diffusion is limited by the elastic stress caused by the stretching polymer chains and by any other stresses that act on the polymer phase. The network is comprised of large macromolecular chains and the solvent phases is of low molecular weight, so that the liquid phase when unconstrained can be highly mobile compared to the network. In a constrained state, where both the polymer and the liquid are enclosed by a contacting boundary, the polymer network and the solvent phase tend to act in conjunction as an incompressible fluid.
In some applications, the tendency to incompressibility has been exploited for pressure wave absorption by constraining a swollen hydrogel within a porous layer, and relying on frictional flow between the hydrogel and the porous layer in order to dissipate compression energy. See e.g., U.S. Pat. No. 5,885,912 to Bumbarger, issued Mar. 23, 1999. These systems act to confine the hydrogel within a porous surrounding layer until the porous surrounding layer becomes subject to significant deformation by, for example, the arrival of a deformation wave at the back-side of an armor fronting plate. The deformation of the porous surrounding layer fractures the confined hydrogel and produces a flow of the fractured hydrogel through the pores of the surrounding layer. The fracture energy and the frictional flow of the highly viscous hydrogel through the pores dissipate some portion of the energy delivered by the arrival of the deformation wave, however the energy of the preceding elastic precursor wave, which produces insignificant deformation, largely passes through the confined hydrogel without attenuation. In the case of a personal armor system, this energy is coupled to the body of the wearer. In a similar application, confined hydrogels are utilized for isolation of blasts arising from spontaneous gas explosions in a mining environment. See Luo et al, “Experimental Study and Property Analysis of Seal-filling Hydrogel Material for Hermetic Wall in Coal Mine,” Journal of Wuhan University of Technology-Mater. Sci. Ed. 25 (2010). In the latter application, the swollen hydrogel acts to absorb blast energy through elastic deformation of the polymer network. This mechanism can marginally operate in the absence of a deformation wave solely through elastic stretching, however the confined nature of the swollen hydrogel maintains a constant percentage of free water in the hydrogel, and eliminates any subsequent dissipation through the frictional flow of exuded free water.
It is known that swollen hydrogel particles under an unbounded compression undergo a viscoelastic deformation which acts to drive at least some free water from the swollen hydrogel polymer network. High-speed compressions indicate that the viscoelastic nature and frictional flow between the free water and the polymer network can produce significant force relaxation. See e.g., Wang et al., “High-speed compression of single alginate microspheres”, Chemical Engineering Science 60 (2005). Significant yielding and deformations up to 50% may occur prior to failure of the swollen hydrogel particle. Generally speaking, the friction coefficient of the swollen polymer network and the free water is proportional to the ratio of the viscosity of the free water and the average mesh size of the gel. For permanently cross-linked polymer networks, the friction can be enormous because the polymer network is a mesh of molecular size. See e.g., Doi et al., “Friction Coefficient and Structural Transition in a Poly(acrylamide) Gel”, Langmuir 21 (2005). As a result, if a swollen hydrogel could be arranged such that frictional flow between free water and the hydrogel network was allowed in an unconstrained flow environment, these frictional losses could be utilized for effective absorption of a shock pressure, such as that arising from a transmitted shock wave. Further, if the exuded free water were allowed to accrue additional energy absorption through subsequent frictional flow, shock pressures could be further mitigated.
Accordingly, it is an object of this disclosure to provide a shock absorbing layer utilizing a swollen hydrogel in a manner that more effectively mitigates shock wave compression energy, so that the coupling of a shock wave compression to the body of the wearer is further reduced.
Further, it is an object of this disclosure to provide a shock absorbing layer utilizing a swollen hydrogel in a manner allowing absorption of shock pressures through frictional flow between free water and the hydrogel network.
Further, it is an object of this disclosure to provide a shock absorbing layer utilizing a swollen hydrogel in a manner that provides for additional energy absorption through subsequent frictional flow of free water exuded as a result of shock pressure.
Further, it is an object of this disclosure to provide a shock absorbing layer incorporating a plurality of contained hydrogel volumes, so that exuded free water may act to disperse shock pressure energy in directions substantially dissimilar to the prevailing shock pressure wave.
Further, it is an object of this disclosure to provide a shock absorbing layer incorporating a plurality of contained hydrogel volumes in mechanical communication and having increasing free volume percentages as displacement from the shock pressure source increases, in order to accommodate flow of exuded free water and increase frictional flow losses.
Further, it an object of this disclosure to provide a shock absorbing layer incorporating a flexible cooling layer comprised of one or more cooling channels in fluid communication with an ambient environment, so that cooling may be provided to a wearer in a high temperature environment.
These and other objects, aspects, and advantages of the present disclosure will become better understood with reference to the accompanying description and claims.